Puppetswar does a rather convincing Greek themed range named 'Temistokles' (Clearly named after Themistocles, the effective commander of the Greeks at the Battle of Salamis). As far as I can see it does shields, modelled along the lines of the Aspis of the Ancient and Classical era Greek hoplites, and a helmet, modelled along the lines of the Corinthian helmet, albeit with transverse crests that were more commonly a symbol of senior ranks (Basically when you think 'Greek Hoplite', it's
that helmet you automatically think of). Finally they have some 'Light Halberds' (With or without guns) that may or may not be of use. The haft of the longer halberds look fairly decent, but the heads are halberd heads - not the thrusting spearhead of a Greek Doru. Depending on how much of a purist you are, you may want to change the spearheads.
http://puppetswar.eu/product.php?id_product=143
http://puppetswar.eu/product.php?id_product=89
http://puppetswar.eu/product.php?id_product=90
I'd also suggest you pick up as many
FW Minotaurs bits as you can get your hands on, they often have quite a few Lambda (Λ - Basically an inverted V) symbols adorning them that are perfect for a Spartan-esque army (Lambda being the first letter of Lacedaemon -Λακεδαίμων - and often depicted on the shields of new citizens in Sparta's army). Asterion Moloc and Ivanus Enkomi (Minotaurs Chapter Master and Chaplain respectively) often come up on eBay for a reasonable price and would be great for a Character kitbash. Moloc's shield seems to be styled along the lines of the Boeotian shield of Classical Greece, while both his and Ivanus Enkomoni's helmet are styled along Corinthian Helmet lines, with Ivanus's sporting an internal skull faceplate. Also, it's worth keeping an eye out for any of
FW's standard Legion helmets with crests if you want to tie the army in by using standard
GW/
FW stuff. Of course, I needn't say that much filing/cutting/sanding off of Minotaurs specific icons will be needed.
http://www.forgeworld.co.uk/Warhammer-40000/Space_Marines/Space_Marine-Infantry-and-Accessories/ASTERION_MOLOC_AND_IVANUS_ENKOMI_OF_THE_MINOTAURS.html
http://www.forgeworld.co.uk/Warhammer-40000/Etched_Brass/ETCHED_BRASS_MINOTAURS_SYMBOLS.html
http://www.forgeworld.co.uk/Images/Product/DefaultFW/xlarge/minotaurstrans.jpg
So, all in all, hope this helps somewhat and I'd like to see where you take the idea. I toyed with it myself as I love the Ancient Greek period of history, but ultimately I settled for only adding slight Greek aspects into my force, rather than the full-on theme you're going for. Anyway, Good Luck!
EDIT: I've also just found some Spartan heads by Hi-Tech Miniatures that look pretty decent:
http://hitechminiatures.com/10/product/info/85
They also do a full set of Spartan Terminators, but at €69.99 ($93.32) you'd probably be better off converting
GW's Terminators (And you'd probably get a better looking result!)